Samsung SSD 850 EVO (2TB) Review

Table of Contents

Introduction & Background

When the Samsung SSD 850 EVO launched in December of 2014, it won our Editors’ Choice award for its class-leading performance, warranty, and software. At the time, this solid-state drive (SSD) was offered in all the popular capacities, including 250GB, 500GB, and a whopping 1TB, and at competitive prices to boot. (We tested the 500GB version, which, at this writing in August 2015, was selling for around $175.)

Since Samsung left very little that could actually be improved about the drive itself, it did the only thing it could to move the product line (and the industry) forward: double the maximum capacity. By releasing a 2TB version of the SSD 850 EVO in the summer of ‘15, Samsung has become the first SSD manufacturer to break the 1TB barrier in a consumer-ready drive, and at price per gigabyte not much out of step with the 500GB and 1TB flavors of the drive. This 2TB model has a $799.99 list price and was selling for about $750 from online e-tailers, with the 1TB and 500GB models at just less than half and a quarter of that, respectively.

Hitting the new-high 2TB capacity, and maintaining the price-per-gig parity, are feats made possible by Samsung's use of high-density 3D Vertical NAND (3D V-NAND) memory. What’s this “3D V-NAND” business? A bit of technical explanation is in order. (Also, if you're new to SSDs, check out our primer Buying a Solid-State Drive: 20 Terms You Need to Know.) Instead of laying NAND modules side-by-side in an arrangement that is planar, the NAND is now stacked vertically into a three-dimensional cube, allowing Samsung to increase capacity—massively—without needing more physical space. In fact, based on disassembly photos we’ve seen, even at 2TB Samsung doesn’t need all the space inside the slim 2.5-inch drive chassis.

This particular drive uses 32 layers of NAND, and Samsung says it can scale that up to 100 layers in the near future, so what might result could be quite interesting. It will be equally interesting to see if Samsung can replicate this drive’s capacity in an M.2-form-factor drive, but we’re not holding our breath. That’s not to say it’s not possible, just that the market for 2TB SSDs outside of the 2.5-inch form factor is probably relatively small at the moment, given the relative scarcity of M.2 interfaces so far. So we doubt it will move to other form factors in the very near future. We certainly hope we are wrong, though.

As far as how the 2TB drive is different from the previous 500GB version of the SSD 850 EVO we reviewed (or the 1TB version, for that matter), it uses a slightly different controller that is able to address the additional NAND and onboard DRAM on the drive. While the lower-capacity drives use a controller named MEX, the new one used on the 2TB drive is dubbed “MHX.” In addition to having more NAND flash, the 2TB drive also doubles the amount of DRAM onboard, to 2GB. The MHX controller also features a three-core ARM processor, whereas the previous MEX-based drives used a dual-core solution.

Like the previous generation EVO drive, the SSD 850 EVO uses TLC NAND flash instead of the MLC variety found in its big brother, the SSD 850 Pro. TLC stands for “triple level cell,” as there are three bits per cell, while MLC stands for “multi-level cell,” with two bits per cell. Having more bits per cell allows for higher-capacity flash, but it also slows down write operations and shortens the lifespan of the drive, which is why TLC-based SSDs are less expensive than MLC-based ones

Since Samsung is well aware of these limitations, it has taken steps to ensure that not only are its EVO drives reliable, but that their write-speed limitations aren’t apparent to the user. For reliability, its new 3D V-NAND is quite robust if gauged by the warranty Samsung is willing to put on it; the company is selling the drive with a five-year coverage plan and offers it with double the rated endurance of the lower-capacity drives: a whopping 150TB over its lifetime. The key to the increased endurance, says Samsung, is the fact that its 3D V-NAND is made on a fairly large 40nm process, as opposed to using a strictly planar layout where the chips have to be continually shrunk to fit more on a die. The large chunk of silicon allows Samsung to make things nice and roomy, and because the dies are stacked vertically, each of them has more room to operate without any interference from neighboring dies.

With regard to the inherent slowness of TLC NAND in write operations, Samsung has tackled that with its TurboWrite SLC caching technology. Though it doesn’t use actual single-level-cell (SLC) NAND modules, the drive reserves a portion of the TLC NAND to be used as a pseudo-SLC buffer for writes. All write operations are sent to the "SLC" NAND and then written to the TLC NAND when the drive is idle, effectively masking the problem of TLC’s slower write operations. If the SLC buffer fills up, then the drive would have to write directly to the TLC NAND, but you would need to be performing a huge write operation (on the order of over 40GB continuously) to see that happen. So the chances of running afoul of the cache limits are slim in most usage scenarios.

All this technology works quite well too, as the 2TB version of this drive has the same rated performance specs as the 1TB drive, which is pretty much as fast as SATA 6Gbps and the AHCI protocol it uses allow. And those claims were borne out in our testing. It’s capable of sequential-read speeds of 540MB per second, and 520MB per second for sequential-write operations. When it comes to random 4K reads and writes, it can hit 98K IOPS reading files, and 90K IOPS when writing.